Phosphorus
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Phosphorus, a solid nonmetallic element that is required by every living plant and animal cell,
symbol P. Phosphorus was discovered in 1669 by the German alchemist Hennig Brand during his
search for the elusive philosophers' stone, an imaginary substance or preparation believed capable
of transmuting baser metals into gold or silver and of prolonging life. A white solid—
phosphorus—that glowed in the dark and spontaneously ignited in air was found by Brand in the
residue remaining after heating evaporated urine with sand and coal. At that time the name
phosphorus was used to identify any luminous substance (from the Greek phosphoros, meaning
"light-bearing").
Properties
Phosphorus is the second member of the nitrogen family of elements of Group VA in the periodic
table. Its atomic number is 15, and its atomic weight is 30.973762.
There are three main types of elemental phosphorus: white (or yellow), red, and black (or violet).
However, the exact number of allotropic forms has still not been determined, although it is
believed to be about 10. The three types have different densities, solubilities, and reacting
abilities. White phosphorus is the least dense (with a specific gravity of 1.82), black phosphorus
is the heaviest (2.25–2.69), and the red form is intermediate (2.20). White is the most reactive
and black is the least.
White phosphorus is a soft, waxy solid. When pure, it is actually colorless and transparent. It is
the ordinary form of the element, and its α allotrope has been chosen as the standard—or
reference—state for thermodynamic calculations. Its melting point is 44.15 °C (111.47 °F), and
its boiling point is 277 °C (531 °F). It is insoluble in water but soluble in organic solvents, such
as carbon disulfide (CS2) and benzene.
The structure of the liquid, solid, and vapor (below 800 °C, or 1472 °F) phases of white
phosphorus consists of tetrahedral P4 molecules. Above 800 °C, some of the gaseous molecules
dissociate to P2.
One of the most fascinating properties of white phosphorus is its reactivity. When exposed to air
in the dark, it emits a greenish light and white fumes are evolved. This glow is due to the
oxidation of phosphorus vapor by the oxygen in the air. Finely divided phosphorus can ignite
spontaneously to yield the "pentoxide" (P2O5)—which actually consists of P4O10 molecules—in a
plentiful supply of air, or the "trioxide" (P2O3)—actually P4O6 molecules—in a limited supply of
air. The high reactivity of the P4 molecule can be explained in terms of its tetrahedral structure.
Each P-P-P bond angle is 60°. This low value represents a tremendous strain within the molecule,
and thus the species seeks to react in order to alleviate this strain.
Hence, white phosphorus should be stored under water. In addition, it should be handled
carefully, as contact with the skin may cause severe burns. The white form is highly toxic.
Black phosphorus is flaky, with a metallic, graphite-like appearance. The structure consists of
double layers, where each P atom is bound to three neighbors. Unlike the white variety, the black
form is very stable in air. In fact, it is very difficult to ignite black phosphorus.
The physical properties of red phosphorus depend on the method of preparation. Commercial red
phosphorus is amorphous, with pyramidal phosphorus linked in a random network. Its melting
point is 590 °C (1094 °F). It does not ignite spontaneously, does not phosphoresce in air, and is
not as dangerous as the white form. However, it still should be handled with care as it can
convert to the potentially harmful white form at some temperatures. When heated, the red form
emits highly toxic fumes as phosphorus oxides are formed. Red phosphorus is fairly stable and
sublimes with a vapor pressure of 1 atmosphere at 416 °C (781 °F).
Occurrence
Large phosphate rock deposits are found in the former Soviet Union, Morocco, and the United
States. In terms of terrestrial abundance, phosphorus ranks 10th, and it is 19th in seawater
abundance. The element ranks 15th in cosmic abundance. Phosphorus is never found free in
nature, but it is widely distributed in combination with various minerals. An important source of
the element is phosphate rock.
Preparation
Relatively little elemental phosphorus is actually produced for consumption as an end product.
White phosphorus can be prepared by the reduction of the calcium phosphate in phosphate rock.
This is accomplished by mixing the rock with coke and silica and heating the mixture in an
electric furnace or fuel-fired blast furnace to a temperature of 1300 – 1500 °C (2370 – 2730 °F),
at atmospheric pressure. As a result of this thermal reduction, P2 vapor is produced. The reaction
can be written as:
Ca3( PO4)2 + 5 C + 3 SiO2 → P2 + 5 CO + 3 CaSiO3.
The vapor is condensed to a liquid, and thus elemental P4 is obtained.
The black form can be prepared by heating white phosphorus under very high pressure.
Alternatively, it can be prepared by heating white phosphorus at 220 – 370 °C (428 – 698 °F) for
a week or more in the presence of a catalyst, such as mercury, and with a seed of black
phosphorus.
Red phosphorus can be produced by heating the white form in the absence of air at 240 °C (464
°F) for several hours.
Uses
In the United States, about 80% of the phosphorus produced is immediately converted to the
"pentoxide" and then to phosphoric acid (H3PO4). The other 20% goes into alloys, organic
intermediates for oil and fuel additives, pesticides, and plasticizers.
Only limited crop production can occur if soils have deficiencies of available phosphorus, and
phosphorus is probably the most critical mineral nutrient for grazing livestock. Cereals and meats
are the major sources of phosphorus in human diets.
Phosphorus is also used in the production of steels and phosphor bronze. In addition, various
phosphates are used in the fabrication of special glasses, such as those used for sodium lamps, in
producing fine chinaware, in baking powder, and for preventing boiler scale and corrosion of
pipes and boiler tubes. Trisodium phosphate is utilized in cleaning compounds and water
softeners.
Red phosphorus is used as part of the coating of safety matches, in pyrotechnics, and in
incendiary shells. It is also employed in the manufacture of tracer bullets, smoke bombs, and
skywriting compounds.
Zvi C. Kornblum
Cooper Union
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How to cite this article:
MLA (Modern Language Association) style:
Kornblum, Zvi C. "Phosphorus." Encyclopedia Americana. 2010. Grolier Online. 24 July. 2010
<http://ea.grolier.com/article?id=0311700-00>.
Chicago Manual of Style:
Kornblum, Zvi C. "Phosphorus." Encyclopedia Americana. Grolier Online
http://ea.grolier.com/article?id=0311700-00 (accessed July 24, 2010).
APA (American Psychological Association) style:
Kornblum, Z. C. (2010). Phosphorus. Encyclopedia Americana. Retrieved July 24, 2010, from
Grolier Online http://ea.grolier.com/article?id=0311700-00
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